1. Field of the Invention
This invention relates to processing of whole crude oils, blends and fractions in refineries and petrochemical plants. In particular, this invention relates to monitoring performance of components in a refinery, especially monitoring performance of a desalter. This invention also relates to optimizing a refinery operation to mitigate fouling.
2. Discussion of Related Art
Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbon-based deposits on heat exchanger surfaces. These deposits often include inorganic materials as well. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus.
Fouling in heat exchangers associated with petroleum type streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of insoluble materials, and deposit of materials made insoluble by the temperature difference between the fluid and heat exchanger wall.
One source of fouling is carryover of brine and solids from a desalter, which will adversely affect downstream equipment. Typically, in a refinery, raw crude oil arrives containing water and salt. Part of the salts contained in the crude oil, particularly magnesium chloride, are hydrolysable at temperatures above 120° C. Upon hydrolysis, the chlorides are converted into hydrochloric acid, which can migrate to the overhead portion of the distillation column and corrode the condensers. To remove the salts, the crude oil is first treated in a desalter. The desalter is a large vessel full of liquid that uses an electric field to separate the crude oil from the water droplets. As it operates best at 120-150° C., it is generally placed within the preheat train. Downstream of the desalter, the crude oil is further heated in heat exchangers, as is known.
Desalters also help to remove insoluble salts and other solids that are often found in raw crude. Corrosion byproducts, such as iron sulfides, are often found in crude oil and may originate from crude oil pipelines, tanker holds and crude storage tankage. Larger particles will settle to some degree during desalting, whereas finer particles may not. The latter are known to contribute to fouling of crude preheat exchangers. Chemical additives, such as flocculants, are often added to desalters to enhance solids removal.
The impact of desalter upsets on downstream heat exchangers is a known problem. The aqueous brine, which contains dissolved salts such as sodium chloride, when carried-over with the desalted oil leads to fouling in the crude preheat exchangers and can contribute to overhead corrosion in the pipestill itself. Desalter upsets, such as brine carryover, can also pass more solids to downstream equipment.
Another source of fouling is asphaltene precipitation due to blending of incompatible crude oils. Most refineries run blends of different crude oils and care must be taken in blending of these crudes to avoid the unwanted precipitation of asphaltenic materials. Though guidelines are available to assist refinery operators to avoid this situation, it occurs nonetheless, with subsequent heat exchanger fouling.
Once heat exchangers are fouled, they must be cleaned to remove the deposits. Otherwise, foulant deposits reduce the heat transfer efficiency, which requires higher fuel consumption in downstream atmospheric pipestill furnaces. Cleaning typically involves removing the heat exchangers from service and hydroblasting the surfaces or otherwise cleaning the surfaces to remove the deposits. The equipment must then be brought back on-line. The frequency of required cleaning is driven by the amount of desalter upsets and asphaltene precipitation occurring in the equipment.
Mitigating or possibly eliminating fouling of heat exchangers can result in huge cost savings in reduced energy usage. Reduction in fouling also increases throughput and reduces maintenance and cleaning expenses.
Currently, desalter performance is not monitored. Most particulate monitoring methods rely on light transmittance to detect particles in fluid streams. However, due to the optical opacity of crude oil, these methods cannot be used. Other methods such as acoustic based methods are highly influenced by temperature and viscosity variations. Thus, in a refinery setting, these methods have a low level of reliability.
There is a need for monitoring the desalter performance, including the operation and output, especially in real-time and on-line. There is also a need for real-time on-line monitoring of incompatibility-induced asphaltene precipitation during crude blending.